If your Schaumburg warehouse has doors that slam shut on their own, a Wheeling manufacturing floor where the exhaust fans do not seem to do anything, or a building where the heating bill is enormous and the space still feels drafty, you almost certainly have an air balance problem — specifically, a make-up air deficiency.
It is one of the most common and most misunderstood issues in commercial buildings, and it is fixable. Here is what building owners and facility managers in the northwest suburbs need to know.
What Make-Up Air Is and Why Buildings Need It
Every commercial building exhausts air. Restroom fans, welding fume extraction, paint booth exhaust, dust collection, kitchen hoods, warehouse ventilation fans, and general building exhaust all pull air out of the building continuously.
That air has to be replaced. If you exhaust 8,000 CFM and only introduce 3,000 CFM of make-up air through the rooftop units, the building runs at 5,000 CFM of negative pressure. The building will find that air somewhere — through door gaps, loading dock seals, roof penetrations, and any crack in the envelope. That infiltrated air is unconditioned, unfiltered, and comes in at whatever temperature it is outside.
Balanced building = supply air roughly equals exhaust air, with a slight positive pressure. Most commercial buildings should run 5–10% positive to keep outdoor air, dust, and moisture from being drawn in.
Symptoms of a Negatively Pressurized Building
- Doors that are hard to open or slam shut — the classic and most reliable sign
- Whistling at door frames and windows
- Cold drafts near entries and loading docks even when the heat is running hard
- Exhaust fans that seem ineffective — a fan cannot exhaust air the building will not let in
- Combustion appliance backdrafting — the most serious symptom, where negative pressure pulls flue gases from water heaters or unit heaters back into the space. This is a carbon monoxide hazard.
- Odors migrating between tenant spaces or from a restroom into a work area
- Excessive heating costs with poor comfort — you are heating outdoor air that entered uncontrolled
- Dust accumulation near the building perimeter
- Ice or condensation at door frames in winter
If you have doors that slam and a heating bill you cannot explain, those are the same problem.
Where the Imbalance Comes From
Tenant changes without HVAC changes
A space designed as warehouse becomes light manufacturing with a welding station and fume extraction. Nobody added supply air. This is the most frequent cause we find in Schaumburg and Wheeling industrial buildings — the exhaust was added, the make-up air never was.
Kitchen hoods sized without matched make-up air
A commercial hood pulling 3,000 CFM needs roughly 2,400–2,850 CFM of tempered make-up air. Where that was never installed, the kitchen pulls air from the dining room, which pulls it from the front door.
Failed or disconnected equipment
A make-up air unit whose burner failed years ago and was simply switched off. The exhaust kept running. Nobody connected the two facts.
Economizer and outdoor air dampers stuck closed
Rooftop units bring in a percentage of outdoor air through an outdoor air damper. Linkages seize, actuators fail, and dampers close. The building loses its ventilation air and goes negative — and this is extremely common on units past ten years old.
Envelope tightening
New windows, new doors, a re-roof, and better seals reduce the accidental infiltration the building was unknowingly relying on. Comfort gets worse after an efficiency project, which surprises owners.
How a Proper Air Balance Investigation Works
- Inventory every exhaust point in the building and measure actual CFM at each — not nameplate ratings, which are usually optimistic by 20–40%.
- Measure supply and outdoor air at every rooftop unit, including verifying that outdoor air dampers actually move.
- Measure building pressure relative to outdoors with a manometer, under several operating scenarios: all exhaust off, normal operation, and peak (all fans plus dock doors closed).
- Check combustion appliances for backdrafting under worst-case depressurization. This is a life-safety test and should never be skipped.
- Document the deficit in CFM and identify what equipment closes it.
Typical cost of an investigation and written air balance report: $1,500–$4,500 depending on building size and number of units.
Remedies and What They Cost
| Remedy | Typical Cost | When It Applies |
|---|---|---|
| Repair/replace outdoor air damper actuators | $600–$2,000 per RTU | Dampers stuck closed — often the cheapest big win |
| Rebalance and adjust existing systems | $1,500–$5,000 | Capacity exists but is misdistributed |
| Interlock exhaust fans with make-up air | $1,200–$4,000 | Equipment exists but runs independently |
| Direct-fired make-up air unit (5,000–10,000 CFM) | $25,000–$60,000 installed | Large untempered deficit, industrial space |
| Indirect-fired MUA unit | $40,000–$90,000 installed | Where combustion products cannot enter the space |
| Add outdoor air capacity to existing RTUs | $3,000–$12,000 per unit | Moderate deficit, RTUs in good condition |
| Energy recovery ventilator (ERV) addition | $15,000–$45,000 | High ventilation load, recovering heat matters |
Costs reflect 2026 northwest suburban commercial pricing and vary substantially with roof structure, gas and electrical capacity, curb and structural work, and permit requirements. Every project of this size starts with an engineered scope, not a guess.
Tempered vs. Untempered Make-Up Air
Bringing in 8,000 CFM of 5°F Chicago air without heating it will freeze pipes and make the space unusable. Options:
- Direct-fired make-up air: Gas burner in the airstream. Very efficient (nearly 100% of the fuel energy goes into the air), lower cost, but combustion products enter the space. Appropriate for warehouses and industrial spaces with adequate dilution; restricted for some occupancies.
- Indirect-fired: Heat exchanger keeps combustion products separate and vented. Higher cost, lower efficiency, required where products of combustion cannot be introduced.
- Energy recovery (ERV/HRV): Transfers heat from exhaust air to incoming air, cutting the heating load 50–70%. Higher first cost, best payback in buildings with continuous high exhaust rates. Worth modeling on any project above roughly 5,000 CFM.
The Energy Case
Owners often frame this as a comfort complaint, but the economics usually justify the work on their own. Heating 5,000 CFM of infiltrated outdoor air across a Chicago heating season represents a substantial gas load — and it enters uncontrolled at the perimeter, where it does the most comfort damage per BTU. Conditioned make-up air delivered deliberately, ideally through an ERV, costs meaningfully less to heat and actually reaches the occupied space.
Add the secondary savings: rooftop units stop running at maximum trying to overcome perimeter losses, so equipment life extends and repair frequency drops. On many of the buildings we evaluate, the balance work pays back in three to six heating seasons — and there are frequently Nicor and ComEd commercial efficiency incentives available for high-efficiency make-up air and energy recovery equipment that improve that further. We help identify and file for applicable rebates.
Frequently Asked Questions
How do I know if my building is negative without hiring anyone?
Crack an exterior door an inch on a calm day with all exhaust running. If air rushes in and the door pulls, you are negative. If it pushes out, you are positive. It is a rough test but a useful first indicator.
Can I just turn off some exhaust fans?
Only if the exhaust is not code-required or process-required. Restroom, kitchen hood, and process exhaust are generally mandatory. Reducing required exhaust to fix a supply problem creates a code violation and an indoor air quality problem.
Is this a code requirement or just comfort?
Both. Mechanical code requires make-up air for exhaust systems above defined thresholds, and requires that combustion appliances have adequate combustion air. Negative pressure causing backdrafting is a code violation and a CO hazard.
Will this help our summer cooling too?
Yes. Negative pressure pulls humid outdoor air into the building all summer, raising the latent load on your cooling equipment. Balanced buildings dehumidify better.
How disruptive is the work?
Damper repairs and rebalancing are typically done during normal hours with minimal impact. A new rooftop make-up air unit requires a crane pick, usually scheduled for a weekend or off-hours, plus structural and electrical work — generally a two to four day project.
Do you offer financing on commercial work?
We offer Sunbit and PowerPay financing options, and we can structure phased scopes so the highest-return items are done first.
Get Your Building Measured
MMDS provides commercial HVAC services including air balance testing, make-up air design and installation, rooftop unit service, and preventive maintenance contracts for facilities in Schaumburg, Wheeling, and throughout the northwest suburbs.
If your doors slam, your exhaust seems weak, or your heating bill does not match your comfort, call (847) 221-6280 or request a commercial assessment. We will measure the building and give you a prioritized scope with real numbers.